Seed dispersal paradox: farther but fewer seeds dispersed by a primate in a human-modified landscape

As habitats change, animal-mediated seed dispersal increasingly depends on how animals respond to altered habitat structure and resource availability. With habitat loss and degradation accelerating across the tropics, understanding how animal movement influences distance- and density-dependent seed mortality is key to promoting forest regeneration. Across a gradient of anthropogenic disturbance in a tropical lowland landscape of southeast Mexico, we assessed how the movement patterns of black howler monkeys (Alouatta pigra) influence their seed dispersal services through GPS tracking, behavioral observations, and seed sampling. Given the species' long gut transit time, we employed a path analysis to examine how movement patterns across spatiotemporal scales directly and indirectly shape seed dispersal distance. By dispersing seeds away from the parent plant at an average distance of 99.5 ± 64.4 m, black howler monkeys likely reduce mortality risks associated with distance- and density-dependent effects. At the smallest scale, step lengths and relative turning angles emerged as key predictors of seed dispersal distance, revealing that goal-oriented movement strategies may enhance their ecological role as seed dispersers. At broader scales, variability in weekly core area size, driven by resource availability, strongly influenced seed dispersal distance. However, low availability of fruiting trees in disturbed areas may limit the black howlers' ability to support natural forest succession. Ultimately, our findings show that animal responses to human habitat disturbance can influence key processes like seed dispersal, potentially affecting forest regeneration and ecosystem resilience, highlighting the need to understand these dynamics to predict and manage long-term impacts of disturbance on biodiversity and ecosystem function.